High speed far infra-red detector and heat seeking control for guided missiles



8 Sheets-Sheet 1 F. E. NULL SEEKING CONTROL FOR GUIDED MISSILES.xsxsgxssxxy May 30, 1961 HIGH SPEED EAR INERA-RED DETECTOR AND HEATFiled Aug. 29, 1946 INVENTOR. FHY E ULL A mvo f HTTKNFYS May 30, 1961 F.E. NULL HIGH SPEED FAR INFRA- RED DETECTOR AND HEAT SEEKING CONTROL FORGUIDED MISSILES 8 Sheets-Sheet 2 Filed Aug. 29, 1946 INVENTOR.

FAY E. /VUL HTTOE/VE'YS ummm..

May 30, 1961 F. E. NULL 2,986,537

HIGH SPEED FAR :NERA-RED DETECTOR AND HEAT SEEKTNG CONTROL FOR GUIDEDMIssTLEs Filed Aug. 29, 1946 8 Sheets-Sheet 3 IN VEN TOR. F197 E. NULLfirme/lfm l F. E. NULL A-RED FOR GUI May 30, 1961 2,986,637 DETECTOR ANDHEAT DED MIssILEs HIGH SPEED FAR INF'R SEEKING CONTROL 8 Sheets-Sheet 4Filed Aug. 29, 1946 EL?- E- INVENToR. Fay E NULL BY gua Hrm/HEY May 30,1961 F. E. NULL 2,986,637

HIGH SPEED FAR INFRA-RED DETECTOR AND HEAT l SEEKING CONTROL FOR GUIDEDMIssILEs Flled Aug. 29, 1946 8 Sheets-Sheet 5 May 30 1961 F. E. NULL2,986,637

HIGH sPEED EAR :NERA-RED DETECTOR AND HEAT SEEKING CONTROL FOR GUIDEDMrssILEs Filed Aug. 29, 194e 8 sheets-sheet e `FHY E /VULL 9 Trae/VEYSMay 3o, 1961 F. E. NULL l 2,986,637

HIGH SPEED FAR INFRA-RED DETECTOR AND HEAT SEEKING CONTROL FOR GUIDEDMISSILES 8 Sheets-Sheet '7 Filed Aug. 29, 1946 CYCL/ A fm CYCZE Z D N AMay 30, 1961 F. E. NULL 2,986,637

HIGH SPEED EAR INTRA-RED DETECTOR AND HEAT SEEKING CONTROL FOR GUIDEDMIssTLEs 8 Sheets-Sheet 8 Filed Aug. 29, 1946 INVENTOR. /CJ//V// du@ZEE? NWETH tat HIGH SPEED FAR INFRA-RED DETECTOR AND SEEKINGv CONTROLFOR GUIDED MIS- The invention described herein may be manufactured andused by or for the Government for governmental purposes without paymentVto me of, any royalty thereon. This invention relates -to adetector forinfra-red rays. Its. characteristic. ofl most value is its high speedaction. Thismakes it suitable for use in explosives carrying guidedmissiles which are military weapons adapted to find and destroy enemysteel plants, refineries or the like. Such plants are suciently intensesources of infra-red raysto make it possible for a missile to directitself into them, provided that the detector and control mechanisms ofthe missile are suiciently yfast and reliable to give suitableadjustments from a distance of say 4 to 6 miles.

This detector can also be applied lto heat mappers, target locators,etc. The invention resides not only in a detector itself, but also inthe means employed for translatingA the detected signals into strongerimpulsesl suitable for directing the movements of such servo systemswith which missile may be provided for the adjustment of itsaero-dynamic surfaces.

One object of the present invention is to provide a heat-seeking devicewhich can detect infra-red radiation at a distance of several miles andwhich will provide proportional control signal in accordance with suchdetection in a form in which it can be used by a suitable guided missileto make a higher percentage of hits upon heat-detected targets than hasbeen possible with previous heat-seeking missiles.

While an infra-red detector, or a heat seeker as it is more commonlycalled, needs to be sensitive, it is the conclusion of' the presentinventor that sensitivity is not the controlling factor in insuring ahit upon the target. Sensitivity is of value so that the missile canpick out the hottest point in the iield of view, however differences inbackground can outweigh the heat given oi by a distant target in thepreviously employed systems in which one part of the eld of view wasbalanced against another part. Previous attempts to improve seekers havebeenl in the direction of increasing their sensitivity.

Because of the very rapid exponential decay of transmitted radiationthrough the water vapor of the atmosphere, an increase in sensitivityalone does not give corresponding increase in performance. Thus, 14 cm.,of condensable water in a path 1 cm. in cross section and 6 miles long(typical bombing conditions) reduce the transmitted radiation to theorder of 6% of its original value by water vapor attenuation alone.Having a range of 12 miles and 28 cm. of condensable water, thetransmitted radiation would be reduced to 0.36 of 1%, or for the samevalue of received energy, and multiplying by 4 to allow for the inversesquare law, the device would have to be 6/0.36X4=67 times as sensitiveto operate on a given target at 12 instead of 6 miles.

Increasing` the sensitivity of a device does not necessarily better itsperformance. For example, consider a device which balances one half ofits field of view against the other, each side. of the half, fieldsubtending an angle 236,637 Patented May 30, i961 of 8 from the seeker.Thus a target 200 feet square.

of one half of the eld of view. Assuming the targetto have an averagetemperature 55 degrees C. above the background, the seeker will receivefrom it approximately 2.28 times as much energy per sq. ft. as from theright half of the field of view. Then if the right half of the held ofview exclusive of the target, radiated exactly as much as the left half,target on the right half of the eld of View would be equivalent to 1.28times the target'V area (at the temperature of the background) added tothe right half of the field of view. The elective radiant energyreceived from the right half of the iield of view would then be(4.85X106+5.12 104)/4.85 106=1.01 times the effective energy radiated bythe left half of the field of view. If the left half of 4the held ofview radiated one part in 100 more than the right side without thetarget, the result would be an exact heat balance between the two halvesof the iield and no matter what its sensitivity, the device would beinoperative.

it is more eflicient therefore, to sac-rice supersensitivity by using ascanning system having a small scanning spot. Thus for example, a 1square scanning spot is 277 feet on a side at a distance of 3 miles andcovers an area of 7.68X104 sq. ft. The target is assumed to have an areaof 4 1O4 sq. ft. and its equivalent area at the same temperature as thebackground is 5.12X104 sq. ft. which covers (5.12X104/7.68 104)=0.67 ofthe scanning spot. This unbalance must be 64 times larger to cause zeroreceived signal in the case of the seeker having the scanning spot, thanfor a balanced field seeker.

In the drawings Fig. 1 is a longitudinal section, partly diagrammatic,of the head of a missile containing my detector.

Fig. 2 is a functional diagram of the detector and scanning circuit usedtherewith. The sensitive element is shown in vertical cross section.

Fig. 3 is a perspective view of the sensitive or mosaic element showinghow the latter is scanned.

Fig. 4 is a vertical section of the mosaic taken on the line `4 4 ofFig. 5.

Fig. 5 is a rear View of the vertical section of the mosaic taken on theline 5-5 of Fig. 4.

Fig. 6 is a rear view of a vertical section of the mosaic taken on theline 6--6 of Fig. 4.

Fig. 7 is a vertical section of the condenser element of the mosaic.

Fig. 8 is a `diagram of an equivalent circuit for a mosaic element andits scanning device.

Fig. 9' is a diagram of the mosaic circuit in its first charging phase.

Fig. 10 is a diagram of an equivalent charging circuit for a mosaiccondenser during the iirst phase.

Fig. 11 is a diagram of the mosaic element charging circuit in itssecond phase.

Fig. 12 is a diagram of the mosaic discharge circuit in its third orrestoring phase.

Fig. 13 is a cross section of a cold generator and electric temperingcircuit for use with said seeker.

Fig. 14 is a diagram of a master switch governing the sequence ofoperations for the phases of the mosaic circuit.

Fig. 15 is a voltage switch diagram showing the effect of thermal bias.

Fig. 16 is ay diagram of an electrical coupler circuit adapted toconnect the mosaic circuit to the other circuits, i.e. missile controlcircuits for example.

Fig. 17v is a diagram of the cyclic switching circuit for producing theproper sequence of functions.

In order to be able to understand the drawings, it is first necessary tobeinformed of the principles upon which the seeker operates. Brieystated, it is that an insulating iilm, say of certain types of Bakelitei.e. phenolformaldehyde synthetic resins in a thickness derived from thedried solutions or varnishes which approaches that of `a mono-molecularlayer, Vhas electrical conductivity which is profoundly inuenced by theadsorption of water vapor upon it. lt has been shown experimentally thata dierence of 40% in the amount of adsorbed water vapor makes adilerence of approximately 10,000 times in the electrical conductivity.In the device, the lhumidity of a circumambient atmosphere is 'kept asconstant as possible, so that the only variable factoris the drying eectupon synthetic resin film which is exerted by the heatl signal itself.Since the amount of lheatY isfvery vsr'riall, insofar as it is receivedin signal form, it will be'understood that stringently controlledconditions must'prev'ailf General arrangement-Ambient temperaturecontrol Referring now to Fig. 1,720 is a cylindrical housing of shellwhich is intended to be attached to the Vforelpart of the missile (notshown). The shell has. a conical nose 19 which extends back to mirror28. The shell 20 has a heavy coat 23 of heat insulating material whichextends completely around it excepting the nose portion 19. The noseportion must transmit infra-red radiation, therefore it is made ofsilver chloride, suppported by metallic gridwork 21 which is anextension of the solid metallic lining 21'. The infra-red rays from thetarget pass through the silver chloride portion to impinge upon a heavycopper concave mirror 28 which is gold plated on its face and which hasa central aperture 29. The mirror 28 reects the rays to a small concavemirror 26, also gold plated, which brings them to a focus at 59. Therays which diverge from the focus pass through the opening 29. Themirror 26 is mounted on the inner end of a cylinder which is preferablymade of tough glass and which contains a water iilling (not shown).Extending from the'mirror 28 so as to be attached substantially to therear of aperture 29, there is a conical wall 30 of a long cylindricalcontainer 31, the function of which is to house a heat sensitive element32 and the electronguns 33. Between the sensitive element 32 and themirror aperture 29 there is a pair of rock salt plates 24, the functionof which is to prevent local turbulences from producing hot spots whichwould affect the mosaic 32. So sensitive is the mosaic to such scatteredradiation, that, the entire space enclosed by the silver chloride nose27, the two copper mirrors 28 and 26, and the rock salt plates 24, thesensitive element 32 and the electron guns 33 is highly evacuatedinitially, watervapor being added later to the chamber between the rocksalt plates 24 and mosaic 32.

In order to cut down thermal instabilityin the interior of shell 20, asecond wall 34 of heat insulating material is provided within the shellat a substantial distance from the sidewalls thereof. The space betweenthe two insulating walls 20 and 34 is used to house apparatus for thethermal stabilization of a cylindrical space within wall 34.

Such thermal stabilization apparatus comprises a cylinder 35 ofcompressed inert gas such as CO2, nitrogen or helium, which togetherwith a remotely controlled expansion valve 36 constitutes a source ofcold. A source of heat is a chamber 37 which contains water and anelectrical heating coil 38. Heat and cold can therefore he used withinthe shell'as will be hereinafter described so that thel temperature andhumidity conditions circumambient within the detector areV substantiallyconstant. A third chamber is provided to have still a closer Yregulatoryeechsaid chamber being the container 39, the exterior surfaces of whichare provided with a multiplicity of radiating fins 40. Within thischamber is a coil 41 through which'gas may be circulated. Outside thecoil but within the,chamber, there is a lling 42 of parain The lling 42is intendedtobe maintainedwithin itvsws/oliYd-liquid phase, preferablyat solid phase; while the changes in pressure produced on the inert gaswithin the chamber 39, which is brought about by change in volume of theparaffin due to changes in composition of the so1id-liquid phase arecarried by a tube 43 which connects chamber 39 and a disc barometer 44.A small drop in pressure closes the contacts of a relay 45, to which thebarometer is connected, thereby allowing electrical current'to ow in thewater heating coil 38.l The water in chamber- 1 37 is prevented frompouring out by a porous wick 46 ,in which the heating element 38 isembedded. As water boils from the wick, its vapor passes forwardly intheshell between the mirror 28 and forward end of the cylindrical wall 34and then divides on each side of a partition formed by a large diametercopper tube 47 which houses the sensitive element compartment 30. Thelatter is provided with the conical tail piece 48 conforming to whichthe after end 49 of tube 47 is curved. By this arrangement a streamlinedpath for the water vapor stream encircling the sensitive element housing30 is provided.y

This stream is also the source of heat tending to keep the paran 42 in astable thermal condition. The purpose of the copper tube 47 is toequalize the water vapor stream from each Vof its sides to the other bymeans of heat transfer. Y

The container 40 with its filling 42 of paralln serves the purpose of acondenser and the heat carried to it by the water vapor must be equal tothe heat carried away by means of the chilling coil 41. A pump 50 isprovided outside the wall 34 and in circuit with coil 41 and a cold airchamber 51 to pump cold air through coil 41. As long as the temperatureof chamber 51 is kept constant, the cooling elect of the coil 41 willalso be constant and a nearly constant water vapor ow from the heatsource 38 to the condenser 40 will result, regardless 0f the amountcondensed due to changes in the ambient tem perature. `Chamber 51 iskept at. nearly constant temperature by a surrounding =bath formed byexpanding CO2 passing from valve 36 through pipe 52 into the space 53and out of the shell through a pipe 54. The release of CO2 is ,under thecontrol of relay controlled valve 36. A second cylinder 55 of CO2, mayalso be controlled by an electrically operated valve 56 which allows CO2to ow from cylinder 55 through a pipe 57, then around the nose 19 bymeans of a pipe 22, and on its way circulating through the water in thechamber 25 and then out through pipe 22a. A cold source 91 (see Fig. 2)is indicated through an opening 29 in mirror 28. Its function is to coolthe mosaic of the sensitive element; its action thereon being describedunder Details of Sensitive Element." It is-to he understood thatbatteries vsuch as battery 58 for water evaporation wiring and othercommon circuit essentials are to be provided where required.

DETAILS OF SENSITIYE ELEMENT Referring to Figs. 2, 4, and 7, a verticalsection of the heat sensitive elementor mosaic 32 is shown in each. Themosaic 32 is built upon an insulating lrn 75 which provides a base ofvery thin nitrocellulose, glass or synthetic resin. In the course ofmanufacturing the mosaic 32 and reading from left as front to right asback, there is deposited by sputtering in vacuo, in course ofmanufacture, through a mica stencil upon the film 7S, a metallic andmetallic salt i'llm of the same order of thickness as the nitrocellulose75, i.e., 5X 10-6 cm. On the front (left side in Fig. 2) the depositedmaterial provides `a plurality of, plates 76 of black antimony trisulde,on the back of the lm 715 is deposited by sputtering in a vacuum acorresponding plurality of metallic gold plates 77. The deposition .isdone in squares about 4 mm. on an edge. Deposition from the front isalways on space left blank from' the rear and viceeversa. 'The lm isheld in a suitable nonconducting frame 96 (Fig. 6) as. are the otherparts of theY sensitive elements which are about Yto be decreed... f l,

The incident heat raysarc" rellected from the large mirror- 28 tothesmalllconcavemirror 26 which brings it to focus as the real image 59.Therays then diverge through the opening 29 in the large" mirror,through the rock salt windows 24 to strikefv the mosaic or sensitiveelement 32. A partition 60 separates the Water vapor around the thintilm of theV mosaic from the evacuated space containing a cathode beam61 which scans a condenser plate 92. The electron guns 33 supply theelectron beam 61. The space enclosed by the conical AgCl nose or cone27, the heavy copper mirror 28 which is plated with gold or other goodinfra-red reflector on the front face and the rock salt plates 24isevacuated. The after part of container 31, i.e., behind partition 60 isalso evacuated. A porous wick 73 is a source of constant temperature,water in the space housing the sensitive element, which space is betweenthe two evacuated spaces.

The condenser plate 92 just referred to is a part of the sensitiveelement on mosaic 32 and is mounted on the glass rear plate 78. There isan air space 79 between film 75 and' plate 92. Glass plate 7S isprovided on its rear surface with a multiplicity of gold honeycomb cells80 which are preferably square and areV so called because they aresurrounded by low gold walls S1. The function of cells 80` is to act asa collector for electrons knocked out of condenser plates 92 by cathodebeam 61. Glass plate 78 acts as the dielectric of the condenser plates92.

The operation of the detector itself, is as follows: The insulating film75 is very thin (of the order of 5 l06 cm.) and readily adsorbs radiantheat energy from the target and from the heat absorbing antirnonysulfide plates absorbing lm 76; this heat energy passes through the lrn75 and heats rthe inner surface of the mosaic 32. To the right or backof the film 75 is a partial mono-molecular layer of water molecules 82V(Fig. 4). The incident heat causes a relatively enormous change in theelectrical conductivity of this layer of Water molecules 82, antimonysulfide plates 76 acting as heat absorbing plates. The charging circuitsof the electron collector gold cells S0 are scanned by cathode beam 61to plate 92 and by induction through metal and glass plates 60 and 7S tonegatively charged plates 77 on the rear of lm 75 and across theadsorbed water layer 82 on the rear of the front absorption plates 76 tothe negative plates 77 to earth and electron gun 33. The incident energyfalling on squares of cells 76 determines the completeness of theadsorbed water layer 82, hence its surface resistance and the chargingcurrent to the collector cells 80. A signal pickot resistor 83 in serieswith a battery 105, is placed in series with the plates 77' and 77,giving a voltage approximately proportional to the condenser chargingcurrent. Metal plate 60 is held gas tight, so that the best pressure ofWater vapor for the adsorbed layer 82 of water molecules may be used forthe film and a good vacuum used for the cathode ray beam 61.

The cathode ray beam 61 is operated in the way that is conventional intelevision transmission. It is formed by electron guns 33 and deflectedby two pairs of opposed plates 84 (only one pair is shown). As the beampasses over one cell or plate 80 to another (Fig. 3) a correspondingsignal is taken from the pick-offr resistor 104. ln the scanningprocess, condenser plates 92 become positive as secondary electrons areknocked out to the walls 81 of collectors 80.

The actual charging and discharging of the several condensers of themosaic of the sensitive element occurs in several phases as will belater explained in detail under the heading entitled DetailedDescription of Seeker Operation: The Mosaic.

In order to pick out the hottest part of the targeta negative thermalbias is used to cancel out all but the strongest signal in the field ofview. As the magnitude of the pickoi voltage from resistor 104increases, an increased output from the amplifier 85 passes over leadsss and 89 to a thermal biaisY regulator 90. The regulator electronicallycontrols thetemperature of a cold source 91, decreasing its temperaturefor an increased signal. This increases the cooling eiecton the adsorbedlayer of Water on the mosaic, acting in opposition to the energy fromthe target, thus acting as a negative thermal bias to cancel out all butthe strongest signal in the field of view. When the scanning beam passesover the mosaic sectionfwhich is under the hottest part of the image ofthe target, a signal is picked up from resistor 104, amplified by anamplier and impressed on a delay network 86. Aiiter several cycles, thevoltage on this network has built up suficently to trip an electroniccoupler 87v which is in series with the resistor and ampli# fier whenVthe-scanning beam 61 passes over the hottest part of the image.

The electronic coupler then generates a voltage equal in polarity andproportional in magnitude to that across the scanning beam deflectionplates at that instant and impresses it across control leads 95. Thisgives proportional control to the output signal, since the voltage onthe cathode ray detlecting plates has to be such as to produce avdeflection of the beam equal to the position of the hottest mosaicreceiver at that instant. The hottest mosaic cell has a position on theimage mosaic corresponding to the position of the hottest target in thefield of view.

General description.-The monomolecular layer The characteristics of apartial monomolecular layer of adsorbed water molecules on the surfaceof an insulator are utilized to produce a very great and rapid change ofthe surface resistance of the insulator with the temperature riseproduced by incident radiation. As has been stated, the surfaceresistance of someV insulators changes by 10,000 fold for a 40% changein the completeness of a single layer of adsorbed Water molecules. Theamount of adsorbed material on a solid surface de'- creasesexponentially with rise in the absolute temperature of the adsorbedlayer. Only a minute change in incident energy on the adsorbed layer isrequired'to produce a detectabie change in the surface resistance of theinsulator on which the film is adsorbed. Since the Van der Waals type ofadsorption occurs almost instantaneously, the speed of response toincident radiation is limited only by the thickness of the insulatingfilm upon which the water molecules are adsorbed.

General descrpti0n.-St0rage principlev of the mosaic Small metalliccondenser plates 92 in Figs. 4, 5, and 6 are about 1-1 mm. square andare stenciled on to the thin insulating film or glass plate 73 byevaporation. Each metallic wall 81 surrounds the glass surface 78.Incident radiant energy adsorbed on plates 76 and conducted to thinreverse sides varies the surface resistance of the insulatingy lm 75surrounding the individual condenser plates 77. This varies the chargingresistance in series with the condenser iilm squares Which form one setof plates 77 of the mosaic condensers, the set of condenser plates 912of opposite polarity being on the rear of the mosaic, stenciled on theglass insulation plate 78. The pairs of plates forming the miniaturecondensers are charged and discharged by cathode ray beam 61, a resistor104 in series with the collector 81, being used as signal pickoi. Theimage of the eld of view is impressed upon the mosaic at all times, andthe incident energy is stored upon each square for a period equal to thetime between successive transversals of the receiving elements by thescanning spot. In this respect it is very similar in action to theiconoscope and picko resistance circuits used in television. The twosets of condenser plates of the mosaic are separated by an `air tightpartition 60 so that proper pressures may be used for the adsorptionlayer on one side of the mosaic and for the cathode ray scanner on theother side.

General description- Thermal bias control: (See Fig. 2) Without thermalbias control there would be no differentiationbetween signals big enoughto completely remove the monomolecular layer. To prevent this and todilerentiate against the weaker signals so that only the strongest comesthrough, a voltage is taken from the signal pickoff resistor 104 inseries with the collector 81. This voltage is used to electronicallycontrol the energy received by the seeker from an artificial cold source91, in such manner as to produce a negative thermal bias that cancelsout the incident energy from all but the hottest target in the field ofview. Thus, the only signal picked up bythe mosaic occurs when thescanning spot is passing over the hottest signal received in the imageon the mosaic. (This corresponds to the target area emitting the mosttransmittable radiation.) .Y General description of the.weken-Proportional control from the position of the cathode rayscanning beam A high impedance pickoff from the deecting plates of thescanning beam is ideal for proportional control. For the polarity andmagnitude of the voltage of the scanning beam deflection plates 84accurately place the scanning spot on a corresponding portion of theimage. The high impedance pick-olf is provided by connecting the controlgrid and cathode of tube 152 of electronic coupler 87 to the plates 84,as shown in Figure 16. As noted above for a given target position, anelectric signal of suicient magnitude to trip the electronic coupler 87is produced when the scanning beam passes over and only one of themosaic elements representing the eld of view. It is therefore onlynecessary to have this electrical signal electronically connect apickoif from the scanning beam deection plates 84 to the output signalleads 95, at the instant the scanning beam is passing over the mosaicelement giving the signal. At that instant of time the voltage pickedofl` from the scanning beam deflection plates is of the correct polarityand magnitude to give almost perfect proportional control. Specificdescription of seeker details.-Change in resistance of monornolecnlarlayer The surface resistivity of certain insulating lms changes by afactor of 10,000 fold for a change in relative humidity of 40%. Theoryand experiment indicate that adsorbed lms on the surface of insulatorsdo not exceed 1 molecule in thickness. An equilibrium exists at thesurface of the film, the number of water molecules that evaporate fromand condense on the insulating lm per sec. being equal. An increase inpressure of the water vapor surrounding the lilm causes an increase inthe number of molecules condensing per sec., and the number of moleculesin the unfilled monomolecular layer increases until the numberevaporating equals the number condensing. An increase in temperature ofthe absorbed molecules-at constant-pressure ofthe surrounding watervapor-causes an exponential rise in their rate of evaporation, and thenumber of molecules in the layer decreases until the number evaporatingis reduced to the number condensing on the surface. For example a changein :1 fold in resistance (which is ample for detection) corresponds (inthe mean position between 50% and 90% relative humidity) toapproximately,

@ein 2m kT DV T0 Y (i) where,

n:number of molecules in adsorbed layer 'n':number of molecules in thevolume of gas -V=vo1ume of gas adjoining adsorbinglsolid i s:surfacearea of adsorbing solid T0:period of oscillation perpendicular to thesurface of adsorbed molecules k=Boltzmann constant T:absolutetemperature n0=potentia1 energy of adsorbed molecule (equals kineticenergy of adsorbed molecule necessary for escape) m:mass of adsorbedmolecule In the range in which Van de Waals forces cause an almostinstantaneous change in the adsorbed layer with temperature andpressure, the exponential term in Eq. l produces a muchgreater changethan the square root term. The exponential term represents the decreasein the number of adsorbed molecules with increase in temperature becauseof the exponential increase in the rate of evaporation from the filmwhen its temperature is raised.

Experimentally it is found that a fractional change of 0.09 in theadsorbed layer may occur for a change of 1 degree C. A fractional changeof 4x106 in the adsorbed layer was required for a detectable signal inresistivity change, or

degrees C. change in temperature that is detectable by change inresistance of lm with adsorbed layer of water vapor.

The energy required to give this small rise of temperature of a thin lmis readily estimated: Assuming: Receiver-3 mm. square and 10-5 crn.thick; specific heat :0.3; density:2.0; and the heat required to raisethe temperature of the receiver 4.45 l0*5 degrees C, 1s,

Heat:Volume x density x specific heat x temperature change Heat:9.0 l0'IX2.0 0.3 4.45 105 :2.41 X 10-11 calories: l .0l X l0*1 joules :1.01 X10'3 ergs.

In the above estimation, dissipation losses were neglected. Since thedissipation loss is zero `for zero rise in temperature, and -for maxiumrise in temperature, the average dissipation loss would be 1/2 the inputfor a linear temperature rise. If the signal is only stored for a timeinterval equal to t-he time constant of the receiver, the temperaturerise would be roughly linear. Considering the dissipation loss, theminimum detectable signal on the above single receiver would be 2.0210"3 ergs. Or the energy required per cm? of image mosaic area is(l/9.0X102) (2.02 10"):2.24 `102 ergs./cm.2/sig nal. If the target areais 3"X3", i.e., 7.6 cm. 7.6 cm. and the collecting mirror is 8" in dia.with an area of and the image area is approximately 7.6X7.6:57.7 cm.2,then the energy required per cm.2 at the mirror per signal is: 2.2410"2(57.7/324):3.99 103 ergs./cm.2/signal. Since a scanning system isused whereby each receiving element stores energy in the intervalbetween transits of the scanning spot across it, the energy requiredwill be the energy per signal, 3.99)(103 ergs/cm.z into the number oftimes the eld of view is scanned per sec. Hence, if the field of view isscanned 50 times per sec., the energy required at the mirror surface is,3.99 103 50:0.20 ergs/crn/sec. required at the mirror surface.

For 1 mm. square condenser plates, a unit mosaic 4 mm. square could beused, which in a 3" x 3" image area, gives 19 unit mosaic areas in a rowand 19 rows, or 19 19:362 unit elements in the lield of view. At 50scans per sec., this ygives `18,100 possible signals per sec. If the eldof view is 19 degrees square, each unit area of the mosaic would beabout 1 degree square.

Thus, ,by sacricing some of the inherent sensitivity. of

across resistance 104.

adsense Q ai thel'device so that it ,is'aboutas sensitivea'stthefbe'stfheat seekers` at present developed, the effective scanningspot'` can`be`reduce`dto an areal degree lsquare in aeld of view19`deg'ree's square, and the entire eld of view can be scanned 50 timesper sec.

Specific description of seeker dermis-Mosaic and scanning circuit.-(SeeFigs. 3, 4, 5 and 6) The thin insulating lm 75 is secured to a frame 96.The center of each mosaic element Vis a condenser plate 77 which isstenciled by evaporation on the right side of the'thin synthetic resinfilm 75 facing the partition`60. The plate77V is surrounded by a squaresurface 209 of the insulating lm 75 with its adsorbed layer of Watermolecules V82. On the'incident radiation side of the insulatingtilm is athin film 76 of metal to adsorb the radiant energy. This heat energy isconducted through the insulating lm 7,5 to the innerv or right surfaceof the film where the adsorbed layer of water molecules is heated.Surrounding the exposed squares2tl9 on the inner surface of the lm is acommon charging plate '77 which is" alsodeposited through a stencil byevaporation. An open space 79 behind the insulating nlm 75 allows rapiddiiusion of' the water molecules in equilibrium with the adsorbed layer.The gas tight partition 69 allows the proper water vapor pressure to beused for the adsorbed layer, and a good vacuum to be obtained on thescanning beam side of the mosaic. The rear set of condenser plates* 92VYare stenciled upon the insulation 78. The outer and inner condenserplates 77 and 92 constitute a setuof` condensers (one at the center ofeach mosaic element) which' may be charged by the action of the scanningbleam 61` in knocking out secondary electrons to the honeycomb collectorcells 80. The scanning beam isfcontrolledfin the same manner as intelevision, so as to sweep 'for instance from position liito 101 intheupper row and to swing from one row to the next as, 1&1 f"1`02'. u A YFig'.N 8` shows' the equivalent chargingV circuit for one mosaicelement. The upper electron beam is not used in Phasel".

The `following phases occur: Y

`Phase (1).-Incident energy from the lield of view isfalling upon theinsulating lm resistor element 75 (with its absorbing metallic film infront, the insulating lnnand the layer of adsorbed moleculesv on therear ofthe film), the element acting as a series resistance in thecharging circuit. In this charging phase a steady divergentbeam ofelectrons 164 covers the rear of the mosaic uniformly. The number ofsecondary electrons emitted to the collector cells 81 by impact of theprimary electrons of the beam 164 upon the composite layer surfaces offilme plate 92 is limited by the rise of potential on plate` 92 toslightly more than that of the honeycomb collector 81. The potential ofplate' 92 is controlled by thercharging current allowed by heatconducted to film 82 by plate 76 as varied by theheat radiation from theeld of view.

Phase (2).-The steady shower of electrons 164 is stopped, and the beam61 begins to scan over the mosiac. The artificial source 91 is now madecolder than the average temperature of the mosaic and the mosaic 32 nowradiates a relatively large amount of energy to it. This is equivalenttoa partial electrical short as the heat from the'A target is completelycancelled and the mosaic 32 is momentarily cooled below its averagetemperature. This allows the formation of a more complete layer ofadsorbed water molecules, and reduces the temperature of the mosaicsuciently to allow the charging of plates 77 to be rapidly completed.The greater the amount of charge required to charge the rear condensercells 92 to the potential of the collector cells 8l)I in Phase 2; thegreater will be the time integral of the voltage p Thus, the greater theincident radiant energy and temperature of 75 and 82 in Phasev l;

the' greater will Ybe its resistance and*v the smaller. the final chargeon condenser plate 92l in this-phase. The srn'aller the charge on thecondenser in Phase l; the greaterv it will bein Phasev 2, with acorresponding larger signal. Thus, the greater the incident radiation inPhase l, the the greater will be the signal when the scanning'beamcompletes the charge of a given mosaic element, i.e., raises thepotential of plate 92 Ato approximately that of the collector cells 81in Phase 2. The coldsource 91 maintains an effective short circuit ofwater'lm resist'- ance 82 during Phase 3, so that the condenser cells 80can quickly discharge to the initial value that theyl had in Phase l'.

Aj more detailed analyses of the mosaic circuits will now be made:

Phase (1).-(See Fig. 9) charging phase In this charging phase, asteady'diverging showerof electrons 164 covers the rear of the mosaicuniformly `so as to complete the charging circuit of the condenserelement. Thefrear'oondenser plates`92 are covered with a composite layersuitable for copious secondary electron emission. At the start of Phasel, condensers 92`are charged to 50' volts above earth; the rearcondenser plates 77 being negative. The electron gun 33, filament 160and accelerating grid 161, gives the electron shower a kinetic energy ofvolts.` Since the collector is at a potential of 150 volts to earth, theelectron shower will have zero potential on reaching the collector andhence does not reach it in appreciable amount. The electron shower isdecelerated to reach plates 92 with an energy of 50 electron volts, butthis is ample to emit secondary electrons from the composite layer onthese plates, (e';g., caesium metal, caesium oxide and metallic silverin the order named). These secondary electrons are drawn to thecollector which is 150 volts above ground, and the potentials of therear condenser plates are raised until a limit is reached, e'.g.,approximately volts, at which time the number of secondary electronspassing from the plate to the collector is just equal to the electronsreceiving by the collector from the elec'- tron shower. All of the rearcondenser plates thus have the saineV potential to'earth at the end ofPhase l but the condenser charges may be quite different, depending'upon the relative resistances 82 during thisk period. (The volt'- ageacrossl the positive and negative condenser plates varies with the`series resistance, but the potential of all condenser plates 92 withrespect to earth is the same.) If upper filmv 82 of Fig. 9 receives moreradiant energy than the lower 82 during this period, more adsorbed watermolecules will be' driven from its surface andits resistance will behigher than that of the upper one. The equivalent'charging circuit for amosaic condenser 80 during Phase 1 is given in Fig. 10. If theconnecting lines representing the cathodeA ray shower, have a lowresistance relative to 82, the condenser charge is controlledalmostentirely by the variable resistanceA 82 which represents the insulating'film and its adsorbed layer of molecules.

Phase' (2) (See Fig. 11.-Scannng. phase) The steady electron shower ofPhase 1' is replaced by a focussed scanning beam withv electrons at thesame potential above earth as in the steady electron shower. An articialcold source receives radiant energy 162, uniformly from all of themosaic element resistors, 82 and 82', causing them to become' relativelygood conductors and allowing the upper and lower condensers to charge upcompletely during Phase 2. The upper condenser with the smaller chargein Phase l (corresponding to a higher temperature) will receive thegreater charge during Phase 2. The pickoi Voltage 163 from 104' isproportional tocharge Q added to a condenser mosaic element during Phase2, i.e.,` proportional to Q=l--Q. where Q` is the charge added duringPhase 1. Thus the larger the incident radiation, the higher the lmresist'- 11 ance will be, the smaller Q and the larger 1-Q and thevoltage picked olf from `resistor 104 when the scanning beam passes`over ya given mosaic element in PhaseZ.

Phase (3) (See Fig. 12, Restoring phase) i The scanning beam is replacedby the electron shower 164 whose kinetic energy on leaving the electrongun 161 is 50 electron volts, and the radiation 162 to the cold source91 from the lm resistors 82 effectively short circuits the resistors 82.VThe collector 81 also has a potential of 50 volts, and since at thestart of Phase 3 the rear condenser plates 92 of the element condensersare more positive than 50 volts, no secondary emission can now occur andthe rear condenser plates will drop in potential until only a few voltshigher in potential than the collector 81, at plus 50 volts to ground.The conditions are now the same as at the start of Phase l and the cycleis repeated.

Specific description of seeker details-Secondary electron emission frombombarded insulation The insulation 78 in Fig. 4, used as a backing forthe rear condenser plates of the mosaic elements, may also emitsecondary electrons under the bombardment of the electron shower inPhase 1. These surfaces do not have any corresponding conductingsurfaces with completed circuit on the front of the mosaic to act asopposite plates of a condenser. Fig. 4 shows that the metallic 'filmheat absorbers 76 face it, but they are insulated on the'front surfaceof the insulator lm 75, and having no electrical connections, can notact as parts of a condenser. Hence, the exposed surface of insulation78, having only'the capacity of an isolated surface, charges up quicklyand completely during Phase l. Since the energy of the electrons in thebeam of Phase 2 is the same as that of the electrons in the shower ofPhase 1, the insulation surfaces will remain charged during Phase 2, andhence will contribute nothing to the pickoff voltage across resistor104, in Fig. 2.

Specific description of seeker details-Heating ofl mosaic by theelectron bombardment.' (See Fig. 4.)

The heat liberated by the electron showers and beam striking the rear ofthe mosaic is relatively large compared to the minute heat signals thedevice is capable of detecting from the target. The danger is that whenthe target image remains at one place on the receiving'mo'saic for alarge number of signals, e.g., at the center of the mosaic, that thisportion of the adjoining partition 60 and rear mosaic insulating plate78 will become relatively hot to the other portions of the mosaic. Thenif the target image changed its position on the mosaic, these previouslyheated regions of long vthermal time constant would give rise to aspurious signals. To preventthis, all mosaic condenser elements receiveexactly the same amount of charge each cycle. Those condenser elementsin the hotter part of the image receive less charge in Phase 1 but morein Phase 2. This charge is then removed from all condenser elements inthe samemanner in Phase 3. Also the impinging electrons in -Phases 1 and2 have the same voltage. Then, the number of electrons striking eachcondenser elementV is theV same, and the average energy of eachcolliding electron is the same for each element, so that the heatingproduced by electron impact is uniform over the entire mosaic.V VTheabove description neglects second order defects in the electron opticsof the beam. As av further safeguard (see Fig. 7) the partition 60between the front and 'rear condenser plates of the mosaic can be madeof heavy conducting metal as illustrated. As shown by the indicateddistribution of charges,rorn the condenser plates 7,7 of the lm 75,those on the rear plates 92 and on the metal partition 60 and insulation78, there need be no loss of capacity by introducing the thick metalsheet 60 between the condenser plates, Since thecopperplate 60 isinsulated, its negative charges opposite the rear condenser plate 92 andinsulation 78 must be equal and opposite to the induced positive chargeopposite the front condenser plates. The heavy copper sheet shields theinsulating film and its adsorbed water layer from an inequality from theheat produced by the impinging electrons on the rear side of the mosaic.

Specific description of seeker details-Cold source, audio frequencymodulation:

Where rapid modulation of the cold source is required, it is constructedas in Fig. 13. Two small, practically point source hot and cold sourcesand 91, respectively, are placed close togetherV and sufficiently farfrom the mosaic to give uniform coverage. The variable cold source ismade by superimposing the eect of the radiation received from the mosaicby a xed constant cold source, and the radiation to the mosaic from avariable hot source. The cold source 91 comprises small opening 126 in ablack body enclosure 127, the heavy walls of which are cooled by the icebath 128 in copper Waterice container 129. Container 129 has a heavylayer of insulation 130 and willmaintain an approximately constanttemperature for several hours. The ice is frozen in the bath by remotecontrol. A relay v133 opens a cock 132 from a small tank of CO2 131. TheCO2 expands through a chiller coil 128 in the copper water-ice con-Irainer 129 around the outer wall of the black body source 127,producing a mixture of ice and water that will remain approximatelyconstant. The rapidly controlled heat source consists of the light fromneon bulb 1-25. The amount of light energy emitted can be madeapproximately proportional to the current. A filter 134 cuts out theinfra-red which has too much lag to be used from the glass bulb. A smallopening 135 in a screen 134 in front of the filter acts as a pointsource. The current through the neon lamp 12S can be controlledautomatically by means later to be shown by varyingV the grid voltage ofa radio tube 136 connected in series with the lamp.

Initial rough adjustment of the cold source can be made so that itapproximately mergesV withthe background for the rst part of Phasel. Ascreen (not shown, and which is not used `during operation) at varioustemperatures of the optical background to be eni countered, covers halfof the eldfof view, the other half being covered by a screen at the sametemperature as the mosaic except for a small opening for radiation tothe j The diverging and scanning cathode fray beams 164 and 61,respectively, can be started, stopped, and the accelerating voltagechanged, by the use of the proper voltages as supplied by a masterswitching device MS. The proper sequence of voltages to produce therequired characteristics for the electron guns and cold source in thePhases l, 2, and 3 may be obtained by arranging pickoff brushes 137 insequence around a drum 138 carrying insulated segments 139 connected toslip rings 140, al cycle being completed every revolution and the drummaking 50 revolutions per sec. Fig. 14 shows the master switching drum138 with its surface described in a plane, and rotating in the directionof the arrow. The slip rings are connected to the metal plates`orinsulated segments 139 which are flush with the surface of the metaldrum. The brushes 137 connect the'vgiven voltage sources Yat ythe.appropriate time *toV the devices 183v of the electron gun part 33.

. is they are to control.A The sequence of the connections is asfollows:

At same time.

(10) Restore voltage on collector- (11) Cut on fun cold seme---" At sameme' Switching connections Fig'.V 17 is a circuit diagram with the mosaicinsertion of a heat detector and shows the over-all arrangement of thecomponents therein, particularly the switching arrangement controlledbythe rotary drum 138 of the element designated MS (Master Switch) inthis figure. In this circuit the electron beam 61 and the diiusedelectron shower 164 may be generated in the two parts 33 and 33' of thedual electron gun which may be housed in a single tube envelope (notshown). |Ifhe mosaic 32 is scanned normally by the electron beam 61. Thebeam passes through the anodes (not shown) of the gun and is deflectedin the usual manner by the beam deflecting plates 874 after negativecharge is applied to the mosaic condenser plates 92 by the electronshower 164. The arrangement of the elements of the circuit are shown inFig. 17 and are described hereinafter together with their electricalfunctions during the various phases of their operation.

Phase Unef-Staring of the shower by the electron gun 33 Lead 172 betweenthe master switch and the relay winding 181 is positively energized by abattery 210 through brushes 137 and slip rings 140 from one of thecontacts 139 of the master switch. The application of positive potentialto the lead 172 energizes the relay winding 181. The energization of therelay winding 181 closes a toggle switch 182 to the left contactthereof, connecting the positive terminal of a battery 183 to the beamaccelerating grid 161 of the gun part 33. The cathode 160 is connectedto the grounded negative terminal of battery A control grid 186 which isassociated with both the acceleratinggrid 161' and with part 33 of theelectron gun has the proper voltage impressed upon it to give thedesired beam intensity when the grid 161' is made positive. The diiusedelecf tron beam or shower 164 is thus started by impressing a positivepotential upon the grid 161. The electron beam shower 164 is appliedsubstantially uniformly over the mosaic and imparts negative charge tothe condenser plates 92 thereof.

The start of Phase two-To stop the electron shower Lead 173 between themaster switch and a relay winding 187 is energized positively by abattery 210 through brushes 137 and slip rings 140 `from one of thecontacts 139 of the master switch. The application of positive potentialto the relay winding 187 opens the toggle switch 182 and removes thepositive voltage from the grid 184 thereby stopping the electron beam orshower 164.

To start scanning beam and to start full cold source A lead 175 betweenthe master switch and a relay winding 188 makes contact with one of thecontacts 139 of the master switch, thereby connecting battery 210through brushes 137 and slip rings 140 to relay winding 188, and therebyenergizes relay winding 188 and moves an arma ture 189 to the left. Thepositive pole of a battery 190 is thereby connected with theaccelerating grid 161 which is associated with the cathode 160 to whichthe negative pole of battery 190 is connected. A control grid 186 14associated with the accelerating grid 161 has such a rvoltage impressedupon it that the proper electron scanning beams intensity is producedwhen the accelerating grid 161 is made positive. At the same time a lead174 makes contact with another one of the contacts 139 of the masterswitch, thereby energizing the relay winding 194 positively from thebattery 210 inclosing the dual armature 195 to the left. This actionconnects a battery 196 across a resistor 197. The resistor 197 isconnected with the grid 198 of the tube 136 and with the negativeterminal of the battery 196. Such action drives the grid 198 of the tube136 negative and stops the ilow of plate current of the tube 136. Thetube 136 when conducting, supplies heat energy to the temperatureregulator in the cold source 91 shown in Fig. 13. The conductivity ofthe tube 136 increases the temperature of the cold source 91 to thedesired degree.

Start of Phase three- To slop the scanning beam 61 A lead 176 betweenthe master switch and a relay winding 200 was energized from one of thecontacts 139 of the master switch and the battery 210i to actuate therelay winding 200, thereby closing the right-hand contact of the relayarmature 189. Such action connects a battery 201 in series with theelectron gun 33 and the accelerating grid 161 thereof, thereby putting anegative voltage on the accelerating grid 161 and stopping the electronscanning beam 61.

To start the electron shower 164 at a reduced voltage A lead 177 betweenthe master switch and a relay winding 202 makes contact with one of thecontacts 139 of the master switch, thereby energizing from battery 211the relay winding 202 and closing the switch 203 to the right. Thebattery 204 is thereby connected in series with the cathode of theelectron gun 33' and the accelerating grid 161' thereof. Such actionstarts the electron shower 164 at a reduced voltage since the battery204 produces less voltage than the battery 183.

To lower the voltage on the collector 81 A lead 178 between the masterswitch and a relay winding 205 makes contact with one of the contacts139 on the master switch at the same time that lead 177 makes a contact139. Relay winding 205 is thereby energized from battery 211 so that theswitch 107 closes to the left. The positive potential battery 105 isthen applied through the resistor 104 to the collector 81. The battery105 is of lower voltage than battery 106. The negative terminal ofbattery 105 is connected to ground and the negative terminal of battery106 is connected to the positive terminal of battery 201. The collector81 is put on a lower voltage by switching in the battery 105 instead ofbattery 106.

To restore full voltage on the collector 81 The lead 179 between themaster switch and the relay winding 207 is energized from one of thecontacts 139 on the master switch and thereby energizes relay winding207 from battery 2111. The energization of the relay winding 207 pullsthe armature 107 to the right to connect the positive terminal of thebattery 106 through the resistor 104 to the collector 81. Such actionconnects the Voltage of battery 106 with the collector 81. The latter isthereby enabled to collect electrons splashed from coudenser plates 92.

To cut of? the full cold source The lead 180 between the master switchand a relay winding 208 makes contact with one of the contacts 139 onthe master switch at the same time as lead 179. The relay Winding 208 isthereby energized. The energization of relay winding 208 pulls dualarmature to the right. Such action connects the leads 88 and 89 from thesignal amplifier 85 across the resistor 197 which is associated with thecontrol gridof the tube 136. A signal amplitude can then vary theelectrical bias on the tube `136, which in turn can Vary the terminalbias produced lby-the cold source 91.

Specific details of seeker.-Thermal bias as rapidly as the electron beamcan supply electrons.

Thus all of the spikesnwould start with the same slopev as the scanningspot moved onto the condenser plates, and the voltage would level ofiEat a constant value when the scanning spot was completely on thecondenser plate.

is necessary to have a pickoi 88, 89 for thermal pips and a delaynetwork 86 incorporated with amplifier 85 (see Fig. 2) that gives aresponse proportional to the voltage on a condenser, i.e., proportionalto Iidt. Since In order to differentiate between the signals A and B itthe output of the amplifier 85 is proportional to the energy only thespike A signal is received by the amplifier. The

hottest portion of the field of View has been selected. The delaynetwork 86 of Fig. 2 stores up the energy of the spike for severalcycles until it is sufficient to trip the electron coupler 87 and take aproportional signal off of the deflection plates 84.

Specific description of seeker details-The electron coupler A simplecircuit for the electron coupler is shown in Fig. 16. The deflectionplates 84 are connected by the electronic coupler to the signal leads95. The output from the delay network 86 in Fig. 2 is impressed upon theleads 141 of the electronic coupler. Leads 141 are across battery 142 inseries with rectifier 143 and resistance 144. The rectifier preventscurrent from flowing through the resistance until the input voltageexceeds that of ,battery 142. The voltage then picked up across theresistance is greatly amplified by tube 145 andV fed intothetranstiormer 146. The grid and cathode of tube 145Hare connected inparallel with the resistance 144. A battery 153 is connected in serieswith a transformer primary winding 146 and the cathode of tube 145. Thesecondary 146' of this transformer is connected to the grid of a tube147. This tube is normally biased to cut'oi by battery 148. An all ornothing effect is obtained, for when the input leads 141 receive avoltage pulse only a small fraction larger than that of battery 142, thevoltage from transformer 146 is suicient to overcomeY a biasing or cutobattery 148, and tube 147 is made conducting. Nearly all of the voltageof a battery 149 is then impressed across a resistance 150. Resistance150 is made larger with respect to the resistance of tube 147 whenactivated by the transformer 146 and the tube grid. A tube 152 having acathode, controlgrid, screen grid and plate is provided. The deflectionplates 84 are connected in series with the control grid 155 of the tube152.v A resistor 150 yand a battery 151 are connected in series with thescreen grid of tube 152. Since the voltage across resistance 150 frombattery 151 is the normal value for Vthe tube 152, the output totransformer 153 and signal leads 95 `from a battery 160 will beproportionall to the input voltage on control grid 155 and cathode 1560itube4V 152 from the cathode ray beam deection plates 84 which isin turnresponsive to mosaic output. It is necessary to use a thermal ratherthan an electrical -bias to eliminate all but the strongest signal, aslarge signals might drive .off all of the adsorbed water molecules fromthe thin insulating receiver films, and these mosaic elements would givethe same response regardless of the relative signal size. Y Y Y Y. Y. Y

Specific description of seeker details.-Magnitude of A pickot voltageThe order of magnitude of the capacity of one of the small condenserelements of the mosaic is,

C=KA/41rd A.

where,

K=die1ectric constant of insulation between condenser plates.

A :area of one condenser plate.

d=thickness of dielectric between plates.

This application of the above formula to the condensers of the mosaic issubject to a considerable error due to edge effect, but the order ofmagnitude is the only value considered. Most of the insulation betweenplates is l water vapor with a dielectric constant of approximately l1.A=l mm'. l mm.=l02 cm?,

l/QX 10'11 3.v99 l03=4.43X 10-15 farads Ifthe change in condenser chargeis that due to volts, Q=CV=4A3 X 10-15 1OZ=4.43 X10-13 coulombs. For18,100 elements per sec. the time of passage of a scanning beam overa'condenser plate would be d=0.20 om. C:

=5.53 10*5 sec.

For `a pickoff resistance of 104 ohms, the pickof voltage Would be 8.01l0 l04=8.01X10"s V. If the minimum detectable signal is 1/10 of thismaximum value, minii mum signal 0.1 8.0l 105=8.01 106 volts, or 8 imicro-volts, which is readily detected. Since the frex quency range ismuch less than that in television, considerably higher values of pickoffresistance Vcould be; usedY with correspondingly higher pickoffYvoltages, withi out thermal noise limitation. Y

Specific description of seeker details- Electrical time constant ofmosaic element: (See 5) The square condenser plate 77, l mm. on aside,is separated from the outer charged plate 77d by the 3 mm. square ofinsulating film 75 with its partial layer of adsorbed molecules. Arelative humidity may be chosen such that the specific resistance of theinsulating lm is =5 107 ohms across the sides of a mm. square ofsurface. The resistance of the film surface between the l mm. square andthe plate is, R=L/A, where L= 1` mm., and A as indicated by the dottedline 209 is approximately 4.5 mm. in effective length. Hence,R=L/A=approx1 mately 5 l07 (l/4.5)=1.1l 10'I ohms. The time constantT=l.l1 107x443 X 1()-15=4.92)1 10"8 `sec.

, AFunctional explanation of cycle of operations -Atthe start of Phase 1the collector 81ispositive with respect to the discharged condenserplates 92 and 77. The electron shower 164 is started by contact 17V2`andrelay 181, and the condenser plates 92Vare quickly brought up to apotential slightly exceeding that of the collector 81 so that no moresecondary electrons leave condenser plates 92 than primary electronsarrive in shower164. Charging ofthe plates 92, however, does not chargeup the .condenser elements consistingY of platesV 9.2A and 77.

ansehe? A condenser cannot be charged byraising :the potential vof :anisolated plate 92; to put appreciablefcharge on .the condenserconsisting of plates 92 and 77, la complete charging circuit must beestablished. The charging circuit is completed through the surfaceresistance of lm 75, on the front surface of which is depositedinfra-red absorbing lm 76. lf an infra-red image is focused on anabsorbing plate 76, the lm 75 in that section ofthe mosaic 32 will havea higher resistance than in Vother sections and thecorrespondingcondenser element with plates 77 and 92 will have a relatively smallcharge on its plates at the end of Phase 1. The condenser plates 92charge through the secondary electrons emitted'to collector 81, throughresistor 104, and the battery 106 to earth, the toggle switch -107having its'right contact closed'during Phasel l. The signal voltage that4occurs across resistance .1104 during the charging of a'mosaiccondenser `element is amplified by 85 whose output leads 88 and 89 arevconnected through switch 195 across the grid resistance 197 Aof'tube196. When the heat signal on the mosaic is too Ylarge and wouldevaporate the water lilm from all .the mosaic elements alike, thuseliminating any diierence in signals from the section under the targetimage and'from therest of the mosaic, tube 196 is blocked byzthe extravlarge negative signal on its grid. This stops the current through thelamp heater unit 125 of the cold source 9.1, fthus in effect providingan increased cold source 'which -acts as a thermal bias to limit thesize ofthe heatsignal in the same way as automatic volume controllimitszthe f size of the electrical signal. The. signal fromamplifier-85 goes to the delay network '86 and can actuate` the couplingcircuit 87, connecting the output leads 95.to.a volt- ;age'proportionalto that across .the deectionfplates184. "No signals are obtained from 95in therst phase, .howl ever, since scanning beam 61 has not ibeen;turned on .and no "deecting voltage is onplates 84,sincetheiconventional scan oscillator voltageis connected to 4.plates84;by -fswitch 189 at the same time beam161 isstarted (starting fconnection for standard scanning voltagernotV shown), at

:the end of Phase l, lead 173 is connectedandy relay 187- is energized,opening switch 182 andstopping the:elec .tron shower y164.

Phase II.-Scanning 'phase Y The charge on the mosaic condenserelements,`92and 77 in Phase I, diered from maximum by an amount V,proportional tothe infra-red radiation absorbed by'the mosaic element. Hence, in PhaseIl when the charge on the condenser elements is completed; the chargeflowing t onto the condenser elements throughresistor104when [thecharging circuit is completed by the scanning beam 561, is proportionalto the infra-red energy -receivedby the given mosaic element. Lead 175is` energized to close z relay 188 to start scanning beam 61. In orderforthe mosaic condenser elements 92' and 77v to'be'complet'ely I.charged, it is necessary to `reduce the resistance of film 75 that is inseries with the charging circuit. 'This lreduc- :tion in resistance isproduced 'by starting the cold source 191. YTo start the cold source 91at the beginningof Phase II, lead 174 is energized, operating relay 194,pulling Aswitch 195 to the left and placing battery 196 across resistor197 driving the grid 198 of tube `196 sufficiently negative tocompletely stop the heating lamp 125 ofcold :source 91. This makes coldsource 91 a maximum. `During APhase Il, signals are picked off yfromresistor 104.as 4the mosaic condenser elements'are scanned. Th'esesig`vnal'voltages are ampliiied by 85, delayed by network 86 :and impressedon the coupling circuit 87Which4 plates a yvoltage across the outputleads 95'proporti0nal tothat vacross the deflection plates 84 attheinstant the 'scanning `Ebeam 61 passes over the mosaic element thatis made'the hottest bythe infra-red image. AmpliiierSS is equipped withan automatic bias `control to depress all signals so that only thestrongest comes through. The delay circuit Icanf 'overcomethe effect ofa1- decoy-'sgnaklastingffor only fone 'cr 'two cycles.

vless heating of mosaic.

`I8 ,Beam 61 is stopped at-the end of Phase II by the energization oflead 176, the activation of relay 200 and opening switch'189.

Phase IIL-Restoring phase To discharge the condenser elements 92 and 77the full cold source is left on to reduce the resistance of lm in serieswith the discharge circuit. Lead 177 is energized to operate relay 202,pulling switch 203 to the right to put the accelerating grid 161 ofelectron gun 331 on the reduced voltage of battery 204. At the same timevoltage is reduced on collector81 by lead 178 making contact with 139,relay 205l closing left contact of switch 107, `connecting the batteryin series with collector 81. Condenser plates 92 will now be at a higherpositive potential than collector 81 and no secondary electrons will beemitted until the potential of plates 92 is reduced to vapproximatelythat of collector 81. The voltage of electron shower 164 was lowered asplates 92 could be discharged by electron shower 164 without electronsecondary electron emission, and the lower voltage produces At the endof Phase Ill just before the start of Phase I, again, lead 179 contacts139,

relay 207 is energized and switch 1017 connects battery right-handcontacts of switch which removes the maximum cold source' by againconnecting leads 88 and 89 -across Vresistor 197 so that tube 196 cannow lbe reguvlated'bythe signaloutput'from amplifier 85 to providethermal bias. Battery 204 for the lowered accelerating grid voltageonelectron gun 33 is automatically discon -nected'when switch 182connects battery 183 in series ywith the grid 161 -atthe start-of Phasel. When relay 202 pulls switch 203 to close the right-hand side contact,it pulls itagains the tension of aspring (not shown) until a catch- (notshown) latches it in position. -When switch '182 closes to the left, itreleases the latch on switch 203 allowing it to open.

The invention claimed is:

flfln an infra-redv radiation detector, a multiplicity of cells arrangedas a` mosaic; in each cell thereof a substantially central condenser, athin insulating iilm sur- 45' rounding said condenser, an adsorbedpartially monomolecular aqueous tilm upon said insulating film wherebythe resistance of the insulating film varies with the completeness ofthe adsorbed layer of water molecules as controlled by incidentinfra-redradiation.

`2. An infra-red radiation detector according to claim l in which theinsulating film is synthetic resin ofthe phenol-formaldehyde varnishtype.

3."In 'an infra-red detector, a mosaic element, a resist- Vance filmbearing a multiplicity of individual cells, said lmbeing adapted to actas one plate of a condenser of said mosaic element a second platebearing a multiplicity of mosaic condensing elements for assisting'thefirst plate to complete the functions of a condenser, the resistance ofthe first plate being in series with thel condenser so completed, a gastight partition separating the two plates, means for maintaining asubstantially constant humidity on that side of the partition containingthe resistance film, so that the vapor pressure of the resistance sideof the partition yis suitable for variation of the resistance by changekof the .number of adsorbed molecules ofthe vapor with incidentinfra-red radiation and means'for maintaining a partial vacuum aroundthe con denser plate on the opposite side of the gas tight partition sothat said condenser plate may be scanned over its individual condenserelements by a cathode ray beam. '4. In an infra-red radiation detector,the combination which comprises a heat-ray-sensitive electrical mosaic,

of the radiation of said cold source to said-mosaic, said meanscomprising an electrical resistance located between said cold source andsaid mosaic and an electronic coupling circuit adapted to use part ofthe electrical output of said mosaic to bias said cold source by meansof the heat generated by said resistance so that the heat input to themosaic from the field of view is cancelled except for the most intenseinfra-red local radiation derived from the field of view.

5. In an infra-red detector suitable for directing guided missiles formilitary purposes, a shell adapted to be attached to the front of amissile, heat insulating material substantially surrounding said shellexcept at the front portion thereof, an infra-red ray-transparent nose,an infra-red sensitive electrical element within said shell, means formaintaining stabilized conditions of humidity and temperature aroundsaid sensitive element and electrical means adapted to use a part of theelectrical output of said sensitive element to thermally bias theinfrared rays received by said element whereby to cancel out allinfra-red signals except substantially the strongest ones.

6. In an infra-red detector, an infra-red sensitive electrical mosaic,said mosaic comprising a multiplicity of condenser plates, a resistancein film form in series with said plates, said resistance being variablewith the incident infra-red radiation from the field of view, aregulatable artificial cold source disposed in the path of the infraredradiation on its way to the mosaic, an electrically resistant heatsource between the cold source and the mosaic, said source being adaptedto receive a part of the electrical output of said mosaic resistances,thereby causing a partial short circuit of said heat source whereby arapid discharge of the condenser elements of the mosaic is brought aboutduring the restoring phase of the operation of scanning said mosaic witha cathode ray beam.

7. In an infra-red ray detector, the combination which comprises a heatray sensitive electrical mosaic, a multiplicity of condensers in saidmosaic, a resistance in ihn form in series with said condensers, anelectron gun, means connected with said gun for at one time showeringall of said condensers and at another time for scanning all of saidcondensers serially with an electron beam having a small scanning spotand a rapid scanning action, said beam serving to connect serially, allof said condensers to earth.

8. In an infra-red sensitive electrical detector, the combination whichcomprises a mosaic element, a multiple condenser therefor, a s eriesresistance in film form connected with said condenser and variable withthe incident infra-red radiation, electron guns adapted to bombardcondenser plates on one side of the mosaic, a multiplecelled electroncollector of conducting mesh mounted in closely adjacent relation to theside of the mosaic which is to be scanned, said collector being adaptedto receive the secondary emission from the condenser plates and to limitthe charging potential of the condenser units.

9. In an infra-red radiation detector, the combination which comprisesan approximately constant temperature boiler, an electrical heating coiltherefor, a condenser, means for providing a streamline flow of vaporfrom said boiler to said condenser, a constant temperature bath Withinthe condenser, said bath containing a material having a liquid phase anda solid phase that can coexist at the temperature of condensing steam, apressure-sensitive device under control of the pressure of gas or vaporat the top of said bath, a relay under control of said device, a coolingcoil in the internal constant temperature bath thru which a coolingmedium from another constant temperature bath is circulated whereby thevapor flow to the condenser will carry heat to it equal to the amountremoved by the coolingcoil and the vapor ow is maintained constantregardless of varying condensation along its path as caused by changingambient temperature. v

10. In combination, an artificialI cold source and means for temperingthe cold from said source whereby to produce the overall effect of acold source with high speed l modulation to create a constanttemperature, said tempering means comprising a hot source emittingradiant light energy comprising a substantial proportion of infra-redrays, a modulating means for said hot source,

an electric heat detector placed in the zone of radiation of said hotsource and of said cold source, said modulating means comprising anelectronic tube of at least three electrodes, one of which is a grid,and an Velectronic cird cuit whereby a portion of the ouput of said heatdetector is diverted to the grid of said tube.

11. The method of detecting a strong infra-red signal which comprisesreceiving saidV signal on an infra-red detector having a substantialextent of field, thermally biasing said detector to cancel out allbackground infrared so that only the strongest signals will come thru,ap-v plying said strongest signals to evaporate a partiallymonomolecular film of water from portions of a synthetic resin film insaid detector, whereby to generate a static charge on said iilm,accumulating said charges on previously charged separate condenser cellsof said detector, scan-` ning the cells after the addition orsubtraction of charge due to the superposition of the film charge on thecon;

.denser charge, amplifying the scanned image, then feeding saidamplified image thru a delay network then thru an electronic coupleradapted to generate an output signal 'having a voltage and polaritysufficient to give said signal proportional control properties when itis supplied to an appropriate electro-mechanical system.

12. A missile head comprising, in combination, an' outer cylindricalshell, an inner shell concentric therewith, vheat insulating materialsubstantially surrounding both shells, a conical nose for said outershell, a light# lray-transparent'apex on said conical nose, a mirror system for focussing the rays admitted through said trans- -parent apex andfor 'projecting them rearward through the inner shell, aheat-ray-detecting mosaic mounted within said inner shell, means forscanning said heat-ray-detecting mosaic, a third shell surrounding saidheat-ray-- detecting mosaic, said shell being partly Vcomposed of amaterial transparent to heat rays, said transparent material sealing thethird shell near the forward end thereof, the space behind thetransparent material and including the scanning means being evacuated ofair and means for keeping constant the ambient temperature and humiditywithin the entire space within the shells except that space which isevacuated.

13. In an infra-red detector, a heat seeker comprising a vheatradiation-sensitive electrical mosaic capable of converting an impressedimage into an electrical signal, an output circuit for said seeker,means for emitting a cathode-ray directed toward said Ymosaic, cathoderay deflecting means for causing the cathode ray to sweep said mosaic, acoupling circuit energized by signal from the mosaic caused by the imageimpressed thereon and transmitting a voltage signal to the seekeroutputV circuit proportional to the voltage across the cathoderaydeiiecting means, saidcoupling circuit having an output circuit andvalso having an input circuit comprising, in series, a rectifier, afirst battery and a first resistor with the coupling circuit inputcircuit voltage impressed thereacross so that no current iiows into theinput circuit until the-input voltage exceeds that of the rst battery inthe inputcircuit, a first input tube having a grid and a cathode acrosswhich the first resistor is paralleled and having a plate, a firsttransformer having primary and secondary windings, a second battery inseries with said first transformer primary winding and with the plateand the cathode of said first tube, a second tube having a plate, acathode and a grid, a grid-biasing third battery in series with the gridand cathode of said second tube and with the secondary winding of saidfirst transformer, a ,fourth battery providing potential for the plateformer primary winding, and said second transformer 10 secondary windingconnected across said seeker output circuit .and providing seeker outputsignals whereby a voltage will be `furnished to the output circuit ofthe seeker as actuated by said mosaic.

References Cited in the tile of this patent UNITED STATES PATENTS2,306,272 Levy Dec. 22, 1942

